I tried to use foundation on two separate occasions. Each time, basic features were broken (or completely missing!) out of the box, and when I tried to investigate, the code was so tangled and awkward that I either abandoned foundation, or implemented the feature from scratch myself.

Bootstrap is pleasant to work with and extend. Each of the javascript components are simple, isolated, and well thought out.

But don't take my word for it!

The "affix" plugin in bootstrap: https://github.com/twbs/bootstrap/blob/master/js/affix.js

The equivalent in foundation: https://github.com/zurb/foundation/blob/master/js/foundation...